Britain's Foreign Secretary William Hague will formally meet Burma's main opposition leader Friday, the second day of his historic visit to Burma.

Hague had a private dinner with Aung San Suu Kyi Thursday evening, ahead of the formal visit. Earlier in the day, he met with President Thein Sein and other top officials in the administrative capital, Naypytaw.

The British official said his message to the Burmese government was: "If you want those sanctions, those restrictive measures as we call them, lifted, then it is very important to show that you are completing this process of reform." He said the release of the political prisoners is one of the main conditions.

Aung San Suu Kyi cautioned Thursday that the democratic reforms started by Burma's nominally civilian government are not "unstoppable,'' and that the military still could block changes. But she expressed cautious optimism.

Her National League for Democracy received official approval Thursday to participate in parliamentary by-elections for 48 seats, set for April 1.

Aung San Suu Kyi also said that this year will show what progress has been made toward genuine democratization in Burma.

Hague is the first British foreign secretary to visit Burma in more than a half-century. His visit is similar to the one U.S. Secretary of State Hillary Clinton made in November. Both visits come after the new government took over in March and started a dialogue with the opposition and ethnic groups.

Aung San Suu Kyi's party boycotted general elections in 2010 because of electoral restrictions that prevented Aung San Suu Kyi from running. The new government has eased those restrictions making it possible for her to run for parliament.

In Burma's 1991 election, the then-ruling military government refused to accept the opposition National League for Democracy party's national election victory.